Contrary to an analog-to-digital conversion (ADC), a digital-to-analog conversion (DAC) refers to a processing or device to convert a digital signal having a relatively fewer digits, i.e., generally less than two digits, into an analog signal that can theoretically have an infinite number of digits.
An example of the DAC includes a processing by a modem. In order to convert computer data into audio frequency tone suitable for transmission along a telephone line, a modem employs a DAC circuit.
FIG. 1 is a circuit diagram illustrating a part of a conventional DAC structure.
Specifically, FIG. 1 illustrates a part of a DAC for electric current adjustment having an improved dynamic performance disclosed in U.S. Pat. No. 6,927,714. A digital signal is converted into an analog signal as a plurality of switches bN−1, bN−2, . . . b0 combine electric currents and output the result.
The DAC having the improved dynamic performance in FIG. 1 adopts a mathematical formula:
            Δ      ⁢                          ⁢              I        D                    I      D        =            1              1        +                              g            m                    ⁢                      R            s                                ⁡          [                                    Δ            ⁡                          (                              μ                ⁢                                                                  ⁢                                  C                  ox                                            )                                            μ            ⁢                                                  ⁢                          C              ox                                      +                              Δ            ⁢                                                  ⁢            W                    W                -                              Δ            ⁢                                                  ⁢            L                    L                -                              2            ⁢            Δ            ⁢                                                  ⁢                          V              TH                                                          V              GS                        -                          V              TH                                      -                              g            m                    ⁢          Δ          ⁢                                          ⁢                      R            s                              ]      
As it is inferred from the above mathematical formula, a conventional DAC has an electric current mismatch of various parameters such as width (W) and length (L) of a transistor, gm, VTH, Cox, or Rs.
As a result, a conventional DAC generally has a limited linearity and speed, and is capable of converting only in a relatively low frequency band.